An epitaxial silicon wafer is a wafer obtained by spraying a silicon source gas on a silicon wafer serving as a substrate to grow an epitaxial layer, and is used for a wide variety of uses such as memory elements, logic elements, and image sensors.
In order to improve the integration degree of those semiconductor elements, the flatness of epitaxial silicon wafers is one of the important factors; accordingly, there is a great need for highly flat epitaxial silicon wafers. Further, in order to make more semiconductor elements from one epitaxial silicon wafer, the whole surface, especially including the edge portions of a wafer (wafer end) is required to be flat in shape. The edge exclusion zone for measurement of the flatness of a wafer surface has conventionally been 3 mm from the wafer edge, it is now progressively reduced to 2 mm, and is even further challenged to 1 mm.
Here, with reference to FIGS. 10(A) and 10(B), the film thickness profile of an epitaxial layer grown on the (100) plane of a silicon wafer will be described.
The <110> direction shown in FIG. 10(B) is a reference crystal orientation. The <110> direction in FIG. 10(B) corresponds to 0° (360°), 90°, 180°, and 270° in FIG. 10(A), whereas the <100> direction in FIG. 10(B) corresponds to 45°, 135°, 225°, and 315° in FIG. 10(A). FIG. 10(A) shows the respective film thickness profiles in the circumferential direction of the epitaxial layer measured at 1 mm, 2 mm, and 3 mm inside the peripheral edge of the epitaxial wafer.
As seen from FIG. 10(A), the epitaxial layer is thinner at the peripheral portion of the <100> direction (peripheral region of the epitaxial surface extending approximately 1 mm to 3 mm from the peripheral edge of the epitaxial wafer), whereas the epitaxial layer is thicker at the peripheral portion of the <110> direction. In other words, there is periodic variation in the film thickness in the circumferential direction of the epitaxial layer at the peripheral portion. This is because the growth rate of the epitaxial layer is lower at the peripheral portion of the <100> direction, whereas the rate is higher at the peripheral portion of the <110> direction. Such a character that the growth rate of an epitaxial layer at the peripheral portion of an epitaxial silicon wafer depends on the crystal orientation of a silicon wafer serving as a base is referred to as the orientation dependence of growth rate. Such growth rate orientation dependence causes a reduced flatness of the peripheral portion of an epitaxial silicon wafer. Further, the difference between the maximum value and the minimum value of the thickness in the circumferential direction of the epitaxial layer is larger in areas closer to the peripheral edge of the epitaxial silicon wafer, as demonstrated in FIG. 10(A). This is attributed to the growth rate orientation dependence being stronger in areas closer to the peripheral edge of the epitaxial layer.
As described above, the growth rate of the epitaxial layer greatly varies periodically in the circumferential direction of the epitaxial layer in areas of the peripheral portion of the epitaxial silicon wafer closer to the peripheral edge, depending on the crystal orientation. Therefore, it is known to be difficult to increase the flatness of the peripheral portion, especially the areas closer to the peripheral edge where the film thickness greatly varies periodically. This phenomenon occurs also in the case of growing an epitaxial layer on the (110) plane of the silicon wafer.
To this date, there are known methods for planarizing the surface of an epitaxial layer, such as a method of increasing the flatness by mirror polishing the surface of an epitaxial layer after forming the epitaxial layer (PTL 1: JP H04-122023 A) or a method of controlling the flow of the source gas supplied for growing an epitaxial layer, in the diameter direction (PTL 2: JP 2000-269147 A).